Self-triggering cryogenic heat flow switch

ABSTRACT

Self-triggering cryogenic heat flow switches are used particularly with cooling systems that require a redundant operation. The self-triggering cryogenic heat flow switch has the simplest possible construction while providing a reliable, maintenance-free mode of operation, and ensures a self-switching connection between a heat sink and an end use device or application that is to be cooled. The heat flow switch includes an outer hollow cylinder  1  that is connected with a heat sink  9 , and an inner body  2  that is arranged coaxially to the outer hollow cylinder  1  and connected to the end use device or application to be cooled. When the heat sink  9  is switched off, a concentric annular gap  4  fixed by spacers is provided between the hollow cylinder  1  and the inner body  2 . The linear thermal expansion coefficient of the outer cylinder  1  is greater than that of the inner body  2 , so that the outer cylinder  1  contracts into thermally conducting contact with the inner body  2  when the heat sink  9  is switched on to provide cooling.

FIELD OF THE INVENTION

The invention relates to self-triggering cryogenic heat flow or heatflux switches that are used particularly with cooling systems thatrequire a redundant operation.

BACKGROUND INFORMATION

In satellite communications technology, maintenance-freerefrigerator-cooled electronic systems (antennas, high temperaturesuperconducting filters, amplifiers) are already being used and will beused increasingly in the future. Such systems must be constructed toprovide a service life of several years. The refrigerators are subjectto wear in operation and, therefore, in order to avoid a system failure,at least one other refrigerator must be provided as a redundancy. Duringoperation, this redundant refrigerator must be thermally isolated fromthe end use device or application, since it otherwise acts as aparasitic heat bridge, and must be thermally coupled with theapplication when the primary cooling refrigerator fails. At the sametime, the defective primary refrigerator must be thermally isolated fromthe end use device or application. The switch-over to the redundantrefrigerator must occur as quickly as possible so that the end usedevice or application does not heat up excessively in the meantime andpossibly have to be temporarily taken out of operation. For this reasonit is advantageous to cool down the redundant cooler or refrigerator ina no-load condition and only then connect it to the end use device orapplication as soon as its cold head temperature is lower than thetemperature of the application.

In order to solve this problem, various conventional devices have beenused, such as active electromechanically or pneumatically operated heatswitches, self-triggering uni-directional cryogenic heat exchanger tubesor heat pipes (U.S. Pat. No. 4,673,030), or gas gap heat flow switchesthat are pumped by cryosorption (U.S. Pat. No. 4,771,823).

The active mechanical systems have provided the best solutionfunctionally to date. They are, however costly to manufacture, requireadditional control electronics, and themselves carry a significant riskof failure.

Uni-directional cryogenic heat pipes are still the subject of intensivedevelopment. They are very costly to manufacture, as either highpressure engineering or cryo-engineering is required to fill them withthe working medium. At ambient temperature they are subject to a highinternal pressure, which requires great tube wall thicknesses.Consequently, these heat pipes have a poor switching ratio and are stillperceptible as parasitic heat bridges even after isolation or separationof the heat contact.

In contrast, it is substantially less costly to manufacture gas gap heatflow switches that are pumped by cryosorption. These switches are alsoself-actuating without an additional control. In order to switch over tothe redundant refrigerator, however, the application and the failedrefrigerator must be heated to the extent that the cryosorption pumpdesorbs sufficient gas to close the heat flow switch of the redundantrefrigerator. A high-temperature superconducting application would haveto be taken out of operation to do this.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a self-triggering cryogenicheat flow switch that has the simplest possible construction whileproviding a reliable maintenance-free mode of operation, and ensuringthereby a self-switching connection between a heat sink and an end usedevice or application to be cooled.

The above object of the invention has been achieved in a cryogeniccooling arrangement including a selectively actuatable heat sink, an enduse application which is to be cooled, and a cryogenic heat flow switchselectively coupling the end use application with the heat sink for heattransfer therebetween. According to the invention, the heat flow switchis a self-triggering cryogenic heat flow switch comprising an outerhollow cylinder that has an inner perimeter surface and that isconnected to the heat sink, an inner body that has an outer perimetersurface arranged coaxially relative to and at least partly within theinner perimeter surface of the outer hollow cylinder and that isconnected to the end use application which is to be cooled, and aplurality of spacers arranged radially between the outer perimetersurface of the inner body and the inner perimeter surface of the outerhollow cylinder. The outer hollow cylinder has a linear thermalexpansion coefficient greater than that of the inner body. A concentricannular gap is formed between the outer perimeter surface of the innerbody and the inner perimeter surface of the outer hollow cylinder and ismaintained by the spacers when the heat sink is not actuated. Thisconcentric annular gap is closed and the inner perimeter surface of theouter hollow cylinder comes into contact with the outer perimetersurface of the inner body when the heat sink is actuated.

The heat flow switch according to the invention functions according tothe principle of thermal expansion. With reference to the startingtemperature T. and the linear dimensions at this starting temperature,the heat flow switch has a switch-on point Te and a switch-off point Ta.These points are defined by the following relationships: $\begin{matrix}{{{D( {1 + {\int_{To}^{Te}{{\alpha_{D}(T)}{T}}}} )} - d} = 0} \\{{{D( {1 + {\int_{To}^{Ta}{{\alpha_{D}(T)}{T}}}} )} - {d( {1 + {\int_{To}^{Ta}{{\alpha_{d}(T)}{T}}}} )}} = 0}\end{matrix}$

The heat sink is connected to the outer hollow cylinder of the heat flowswitch. The device or application to be cooled is connected to the innersolid or hollow cylinder of the heat flow switch. While cooling, theouter hollow cylinder contracts until, at the switch-on point, its innerdiameter reaches the outer diameter of the still warm inner solid orhollow cylinder. A heat transfer is established, whereby the inner solidor hollow cylinder and the outer hollow cylinder continue to cooltogether to a temperature below the switch-off point. The compressivestrain between the two parts provides a reliable heat contact with lowheat transfer resistance.

The heat contact opens when the outer hollow cylinder heats up above theswitch-off point. The device or application to be cooled is thermallydecoupled from the heat sink as the heat sink continues to heat up.

These heat flow switches can be constructed with great precision withthe aid of the above mentioned relationships once the temperaturedependency of the linear thermal expansion coefficients of the materialsselected for use have been dilatometrically ascertained. If thediameters D and d are freely selectable, the switch-on point and theswitch-off point of the heat flow switch can be freely selected in broadranges. If one of the diameters is predefined, then either the switch-onpoint or the switch-off point can be freely selected.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in greater detail, based on thefollowing example embodiments, with reference to the accompanyingdrawings, wherein:

FIG. 1 is a lengthwise sectional view showing a heat flow switch of afirst embodiment according to the invention;

FIG. 1A is a lengthwise sectional view showing a heat flow switch of asecond embodiment according to the invention;

FIG. 2 is an enlarged schematic cross-sectional view showing a detailportion along a section plane II—II in FIG. 1; and

FIG. 3 schematically shows an application of the heat flow switches withtwo refrigerators.

DETAILED DESCRIPTION OF PREFERRED EXAMPLE EMBODIMENTS AND OF THE BESTMODE OF THE INVENTION

The heat flow switch according to the invention comprises an outerhollow cylinder 1 and an inner body 2. In FIG. 1, the inner body 2 is asolid body, while FIG. 1A shows an alternative embodiment in which theinner body 2 is a hollow cylinder. As shown in FIG. 2, spacers 3 ensurean even annular gap 4 between the inner body 2 and the outer cylinder 1.For practical considerations, at least three spacers 3 are evenlydistributed about the circumference of the inner body 2 in longitudinalgrooves 5. The spacers 3 consist of an elastic material such as, forexample, nylon or Teflon.

The hollow cylinder 1 is advantageously made of copper and thuspossesses a linear thermal expansion coefficient α_(D)=17 ppm/K and athermal conductivity λ=401 W/(m*K). The inner body 2 consists ofmolybdenum with an expansion coefficient of α_(d)=5.1 ppm/K and athermal conductivity λ=138 W/(m*K). These values are referenced to theambient temperature.

FIG. 3 shows two refrigerators 9 with their dreves 8, whereby onerefrigerator 9 is switched to be redundant. The inner bodies 2 of theheat flow switched are connected by respective flexible thermalinterfaces 6 to the device or application 7 to be cooled. The hollowcylinders 1 are thermally conductingly coupled to the refrigherators 9.

The essential functional components of the heat flow switch can beproduced by a precision mechanics manufacturer so as to provide verygood reproducibility of their characteristics.

What is claimed is:
 1. In a cryogenic cooling arrangement including aselectively actuatable heat sink, an end use application which is to becooled, and a cryogenic heat flow switch selectively coupling said enduse application with said heat sink for heat transfer therebetween, animprovement wherein said switch is a self-triggering cryogenic heat flowswitch comprising an outer hollow cylinder that has an inner perimetersurface and that is connected to said heat sink, an inner body that hasan outer perimeter surface arranged coaxially relative to and at leastpartly within said inner perimeter surface of said outer hollow cylinderand that is connected to said end use application which is to be cooled,and a plurality of spacers arranged radially between said outerperimeter surface of said inner body and said inner perimeter surface ofsaid outer hollow cylinder, wherein said outer hollow cylinder has afirst linear thermal expansion coefficient, and said inner body has asecond linear thermal expansion coefficient that is less than said firstlinear thermal expansion coefficient, and wherein a concentric annulargap is formed between said outer perimeter surface of said inner bodyand said inner perimeter surface of said outer hollow cylinder and ismaintained by said spacers when said heat sink is not actuated, and saidconcentric annular gap is closed and said inner perimeter surface ofsaid outer hollow cylinder comes into contact with said outer perimetersurface of said inner body when said heat sink is actuated.
 2. Theimprovement in the cryogenic cooling arrangement according to claim 1,wherein said inner peripheral surface of said outer hollow cylinder hasa temperature dependent inner diameter identified as D, said outerperipheral surface of said inner body has a temperature dependent outerdiameter identified as d, said first linear thermal expansioncoefficient of said outer hollow cylinder is identified as α_(D), saidsecond linear thermal expansion coefficient of said inner body isidentified as a,, said concentric annular gap is formed upon heating toand above a switch-off temperature identified as Ta, and said concentricannular gap is closed and said inner perimeter surface of said outerhollow cylinder contacts said outer perimeter surface of said inner bodyupon cooling to and below a switch-on temperature identified as Te, andwherein, with reference to a starting temperature identified as To, bothof the following equations hold true: $\begin{matrix}{{{D( {1 + {\int_{To}^{Te}{{\alpha_{D}(T)}{T}}}} )} - d} = {0\quad {and}}} \\{{{D( {1 + {\int_{To}^{Ta}{{\alpha_{D}(T)}{T}}}} )} - {d( {1 + {\int_{To}^{Ta}{{\alpha_{d}(T)}{T}}}} )}} = 0.}\end{matrix}$


3. The improvement in the cryogenic cooling arrangement according toclaim 1, wherein said spacers are respectively filaments of a syntheticelastic material.
 4. The improvement in the cryogenic coolingarrangement according to claim 1, wherein said spacers are respectivelyfilaments of nylon or Teflon.
 5. The improvement in the cryogeniccooling arrangement according to claim 1, wherein a plurality oflongitudinally extending grooves are provided in and uniformlydistributed about a circumference of said outer perimeter surface orsaid inner perimeter surface, and said spacers are respectively receivedin said grooves.
 6. The improvement in the cryogenic cooling arrangementaccording to claim 1, wherein said outer hollow cylinder comprisescopper and said inner body comprises molybdenum.
 7. The improvement inthe cryogenic cooling arrangement according to claim 1, wherein saidouter hollow cylinder consists of copper and said inner body consists ofmolybdenum.
 8. The improvement in the cryogenic cooling arrangementaccording to claim 1, wherein said inner body is a hollow cylindricalbody.
 9. The improvement in the cryogenic cooling arrangement accordingto claim 1, wherein said inner body is a solid cylindrical body.
 10. Theimprovement in the cryogenic cooling arrangement according to claim 1,further comprising thermally conducting members that respectivelyconnect said outer hollow cylinder to said heat sink and connect saidinner body to said end use application.
 11. A self-triggering cryogenicheat flow switch comprising an outer hollow cylinder that has an innerperimeter surface, an inner body that has an outer perimeter surfacearranged coaxially relative to and at least partly within said innerperimeter surface of said outer hollow cylinder, and a plurality ofspacers arranged radially between said outer perimeter surface of saidinner body and said inner perimeter surface of said outer hollowcylinder, wherein said outer hollow cylinder has a first linear thermalexpansion coefficient, and said inner body has a second linear thermalexpansion coefficient that is less than said first linear thermalexpansion coefficient, and wherein a concentric annular gap is formedbetween said outer perimeter surface of said inner body and said innerperimeter surface of said outer hollow cylinder and is maintained bysaid spacers when said outer hollow cylinder is at or above a switch-offtemperature at which said outer hollow cylinder differentially thermallyexpands away from said inner body, and said concentric annular gap isclosed and said inner perimeter surface of said outer hollow cylindercomes into contact with said outer perimeter surface of said inner bodywhen said outer hollow cylinder is at or below a switch-on temperatureat which said outer hollow cylinder differentially thermally contractsinto contact with said inner body.
 12. The self-triggering cryogenicheat flow switch according to claim 11, wherein said inner peripheralsurface of said outer hollow cylinder has a temperature dependent innerdiameter identified as D, said outer peripheral surface of said innerbody has a temperature dependent outer diameter identified as d, saidfirst linear thermal expansion coefficient of said outer hollow cylinderis identified as α_(D), said second linear thermal expansion coefficientof said inner body is identified as α_(d), said concentric annular gapis formed upon heating to and above a switch-off temperature identifiedas Ta, and said concentric annular gap is closed and said innerperimeter surface of said outer hollow cylinder contacts said outerperimeter surface of said inner body upon cooling to and below aswitch-on temperature identified as Te, and wherein, with reference to astarting temperature identified as To, both of the following equationshold true: $\begin{matrix}{{{D( {1 + {\int_{To}^{Te}{{\alpha_{D}(T)}{T}}}} )} - d} = {0\quad {and}}} \\{{{D( {1 + {\int_{To}^{Ta}{{\alpha_{D}(T)}{T}}}} )} - {d( {1 + {\int_{To}^{Ta}{{\alpha_{d}(T)}{T}}}} )}} = 0.}\end{matrix}$


13. The self-triggering cryogenic heat flow switch according to claim 1,wherein said spacers are respectively filaments of a synthetic elasticmaterial.
 14. The self-triggering cryogenic heat flow switch accordingto claim 1, wherein said spacers are respectively filaments of nylon orTeflon.
 15. The self-triggering cryogenic heat flow switch according toclaim 1, wherein a plurality of longitudinally extending grooves areprovided in and uniformly distributed about a circumference of saidouter perimeter surface or said inner perimeter surface, and saidspacers are respectively received in said grooves.
 16. Theself-triggering cryogenic heat flow switch according to claim 1, whereinsaid outer hollow cylinder comprises copper and said inner bodycomprises molybdenum.
 17. The self-triggering cryogenic heat flow switchaccording to claim 1, wherein said outer hollow cylinder consists ofcopper and said inner body consists of molybdenum.
 18. Theself-triggering cryogenic heat flow switch according to claim 1, whereinsaid inner body is a hollow cylindrical body.
 19. The self-triggeringcryogenic heat flow switch according to claim 1, wherein said inner bodyis a solid cylindrical body.
 20. The self-triggering cryogenic heat flowswitch according to claim 1, further comprising thermally conductingmembers respectively connected to said outer hollow cylinder and to saidinner body.